Shot peen inspection technique



J. G. ROBERTS 2,958,925

SHOT PEEN INSPECTION TECHNIQUE Filed May 5, 1959 ARC HE IGHT (we/45s) Il l I l I i I I I l I I g I l I I l l I EXPOSURE TIME (Mm/um) 3 INVENTORJAMES G ROBERTS ATTORNEY United States Patent Office 2,958,925 PatentedNov. 8, 1960 SHOT PEEN INSPECTION TECHNIQUE James G. Roberts, Warren,Mich., assignor to General Motors Corporation, Detroit, Mich., acorporation of Delaware Filed May 5, 1959, Ser. No. 811,119

Claims. (CI. 29-90) This invention relates generally to shot peening ofmetal parts, and more particularly to the method of determining when apart has received full coverage or has been fully treated by the peeningprocess.

Cold working of the outer skin of metal parts by peenj ing has beenknown for several centuries. Peening with tions are uniformly dispersedand the skin has had full coverage, the fatigue life of the article isgreatly increased as to twisting and bending resistance. If the articleis over peened, however, minute cracks occur in the surface due to thefact that the surface is stretched so greatly that the ductility isexhausted. These cracks are very likely to start a spreading fatiguefracture under even very low stresses.

As explained in chapter 2 of the publication entitled Shot Peening bythe American Wheelabrator and Equipment Corporation, Second edition,1946, the relationship between the fatigue life of an article and thestress applied is a geometrical one and when the stress reaches acertain critical value for the material concerned, a small increase instress results in a very large decrease in fatigue life. Shot peeningraises the amount of initial stress required to reach this criticalvalue. Full coverage of the article by shot peening is very much desiredsince the final degrees of the process leading to full coverage mayincrease the fatigue life as much as several hundred percent.

Since full coverage is highly desired but over peening is veryundesirable, the problem of determining this point of full coverage isvery crucial. This problem has caused considerable difiiculty whendealing with high hardness materials.

Accurate determination of this point of full coverage not only helpsprevent over peening, but also aids research workers who have haddifficulty in the study of high hardness items such as bearings. This istrue since one variable property may not be studied exclusively when theproperties are spuriously altered depending on the degree of fullcoverage obtained in the peening process.

Furthermore, it has been noted that there is a time interval between themoment at which full coverage is first reached and that moment when overpeening begins. It is therefore desirable, for low cost productioneconomy, to be able to determine the minimum time at which full coverageis reached when using high hardness materials.

It is therefore an object of this invention to provide a method toenable one to determine when full coverage of a metal article by shotpeening has been reached. A further object is to provide a method toenable one to readily determine this time on metals having surfaces of ahardness above Rockwell C(RC)55. More specifically, it is an object ofthis invention to set forth a procedure to determine when full coverage,by shot peening has been obtained for high hardness steels. These andfurther objects will become more apparent from a reading of thedescriptive matter to follow when read in connection with theaccompanying drawings wherein:

Figure 1 is a plan view of a metal test strip shot peened in accordancewith the present invention;

Figure 2 is an elevational view of Figure 1 illustrating the arc heightof the test strip; and Figure 3 is a graphical representation of therelationship between strip arc height and exposure time to shot peeningof two strips in accordance with the present, invention.

Broadly, the method in accordance with the present invention involvesthe determination of full shot peening coverage of a high hardnessarticle by the use of test strips which are dimensionally equivalent.For convenience, standard Almen test strips may be used along with arelationship which has been discovered as existing between the peeningcharacteristics of soft and hardsurface metals.

The time required for full shot peening coverage of a production itemhaving a hardness of RG55 or greater is found by utilizing twodimensionally equivalent test strips: one of the material and having ahardness equivalent to that of the production item above, and the otherhaving a specific hardness in the range of RC44-50. The full coveragetimes are obtained for these strips in the manner well known in the artand explained more fully hereinafter, and from these two times a ratiohaving a value greater than 1 is set-up. This ratio is then multipliedby the full coverage time required for shot peening a production itemequivalent to the one above, a

but having a specific hardness equal to that of the test strip in therange of RC4450. This latter time may be conveniently found by visualdetermination or by calculations based on the data which have beencollected in the past on materials of this hardness and which are wellknown. to those having ordinary skill in the art, The numerical productobtained from these factors is equal to the time required for shotpeening the high hardness production items to full coverage.

The process involved may be understood more readily in the light of thefollowing background information.

The method of treating metals by shot peening has been studied as ascience and one result of this study has been the development of Almentest strips to be used for shot peening experimentation. The industryhas adopted these test strips as standards. The two strips developed byAlmen are labeled A and C and the specifications of these strips aregiven in the chart below.

It is seen that the only difference between the two is thickness, thusproviding a test strip for both high intensity and low intensitypeening. For convenience,

the Almen test strips were utilized in the conductance: of experimentsassociated with the present invention, but

it is obvious that test strips having dimensions differing from those ofthe Almen strips will Work very well in the proposed method as long asthe dimensional characteristics are alike, i.e., that the strips aredimensionally equivalent for shot peening purposes.

Almen Test Strip When these test strips such as strip in Fig. 1 are shotpeened, the compressive forces setup on the peened surface 12 cause thestrip to bend in an opposite direction creating an arc height as shownby the arrow 14' in Figure 2.- This bending continues until a crucialpoint is reached at'which time the arc of the strip no longer changes.This complete cessation in change holds true if theshot is uniforminsize and other conditions, such as velocity, are ideal. Under conditionsas normally encountered, however, the cessation'in bending is not quiteso 'clearly pronounced, but the full coverage point may still be readilydetermined as the point of drastic decrease in rate of arc heightchange. This point in the peening process at which the arc heightincreases negligibly with further peening is known as the fullcoverage'point, and the arc height is substantially at its maximum valueat this time. This point may be determined as shown by the -x" marks onthe respective curves in Figure 3. The corresponding test strips, a andy; for each curve as shown in'Figure 3 are the relatively soft (RC44-50)and the hard (RC 55) strips, respective'ly.

Inproduction operat ons, the only conditionwhich is allowed to be avariable-is time, since velocity of shot is easilycontrolled using theair-pressure or'cent rifugalwheel-thrower methods of propulsion; and thesame shot iS' recircuIated continuously through the short throwingmachine thus involving no variable in overall shot size.

The time of peening is related to are height of the test strips todetermine at what time full coverage is accomplished.

This point of full coverage may be readily determined for metal articlesother than test strips, if the articles have a hardness between aboutRockwell C44 and Rockwell C50. The time maybe determined by eithervisual means or by estimation calculations based on the data which havebeen collected in the past on materials of this specific hardness. Thepresent invention utilizes this 7 factor in the peening determination ofhard surfaces as described hereinafter.

Many production items today possess hard surfaces, such as areencountered with high hardness steels, of Rockwell C55 and above.Although it is equally desirable to shot peen surfaces of this type,until now there has been no easy, inexpensive, reliable method ofdeterminging the time of exposure corresponding to full coverageof thearticle. Visual means are not really ade quate to determine the stoppingpoint for surfaces having a hardness of about Rockwell C55 or harder.Even under moderate magnification (20x), visual observations areinadequate and unreliable to determine if the surface has beencompletely covered by peening or if the surface has been over-peened.Furthermore, even if it were possible using high power magnificationapparatus, this type of apparatus, if used in production, would be,cumbersome, time consuming, and costly.

The method which has been invented to overcome this difiiculty utilizesrelated properties found existing between the high hardness materialsand the materials for which the time of peening may readily bedetermined. Two physically equivalent Almen test strips, both of eitherthe A or C type, are first heat treated. One strip is hardened to ahardness within the range of Rockwell C44-50 and the other strip isbrought to the hardness of the production item which may be Rockwell C55or above. These two strips are shot peened under like conditions and theseparate times necessary to bring about full coverage are related toobtain a ratio. Full coverage is obtained when the arc height of thealrnen strips, as measured by a standard No. 2 Almen specimen gauge,ceases to change with further peening. The time required for fullcoverage .of the hard metal surface such as a high hardne s steel isconsider y longer than. h

4 required for the softer material of RC44-50. The ratio of the highhardness time to the RC44-50 time (high hardness time) (RC44-5O time)may be called the Almen stripfaetgr (ASF).

(Ttsyl (1) (Ttsa) where Tt s'y is the tirrre g required to shot peen tofull coverage the test strip having a high hardness (RG55 or above), andTtsa is the time required to shot, peen to full coverage the test striphaving a hardness in the range of RC44-50.

The next step is to determine the time required for full coverage of theproduction item if it had a hardness in the range of RC44-50 and'equalto that of the test strip a. (For convenience, call the peening timerequired for the productionitem of this hardness Ta.)

This can be done by estimation on the basis of existing data whichhasbeen collected due to much past experimentation with metals of thishardness, or' by actual- 1y imparting a hardness of RC445O to thestandard item or; an item having an equivalent peening area, and thenvisually determining the time necessary for full coverage" when shotpeening the item'under equivalent production peening conditions.

It has been found that the time required for obtaining full'coverage ofthe high hardness production item (Ty) is a multiple of the timerequired for the physically identical Rockwell C44-SO article (Ta) andmay be accurately found by multiplying the following factors:

, y) (2) (Ttsa) Obviously this mathematical set-up of the factors isarbitrary and may be altered without departing from the scope of theinvention'as long as the true relationship is t (A (T retained. Asample'deter'mination as actually found by this method is given below. I

Example Two C Almen test strips were hardened to Rockwell C62 andRockwell C50, respectively, and were given a shot peening treatment withSAE 230' chilled iron shot at 26 psi. air pressure, i.c., correspondingto the nominal Almen. intensity of 0.010A. The ratio of the time re-.quired for full coverage as determined by means of.

ASF

mission gear with a hardness of RC62. Since the time 7 required for thissame gear having a hardness of RCSO (Ta) can be readily found bycalculations or by peening and visual determination, this time (Ta) isnow simply multiplied by theASF. previously'determined. Thetime of 16minutes is used for (Ta).

Therefore:

Ty=TGEAR=(ASF) (Ta) (2.7) (16) :43.2 minutestime required for shotpeening to full coverage a steel gear with a hardness of RG62. This isjust one example of how this new and useful method may be quickly andeasily used to obtain a reliable and economical determination of thetime required for full shot peening coverage of a hard-surfaced article.

Various changes and modifications of the embodiments of my invention asdisclosed above may be made by those skilled. in the art withoutdeparting from the principles and spirit of my invention asset'forthin-the appendedclaims.

me hod mis s stniin is. fu l 1 1 .ps inaw emsei for a high hardnesssteel article comprising the steps of shot peening test strips toexperimentally determine the ratio of the shot peening time required fora test strip having a hardness of Rockwell C44-50 and the time requiredfor a test strip having a hardness equal to that of said high hardnesssteel article, and shot peening the high hardness steel article for atime proportional to said ratio.

2. A method for shot peening a high hardness article with a hardness ofRCSS or above to full coverage comprising the steps of peening a teststrip having a hardness equal to that of said high hardness article tofull coverage, peening a physically similar test strip having a hardnessof Rockwell C44-50 to full coverage, and finally peening the highhardness article for a time calculated from the ratio of the test striptimes and the time normally required to peen under equivalent conditionsa similar article having a hardness of Rockwell C44-50.

3. A method for determining the minimum full coverage time for shotpeening a high hardness steel article comprising the steps of shotpeening to full coverage a test strip of the same high hardness steeland a test strip having a hardness of Rockwell C44-50, and then shotpeening said high hardness steel article for a time equal to the productof the time required for fully shot peening under equivalent conditionsa steel article with a hardness of Rockwell C44-50 and a multipledetermined from the ratio of the times required to shot peen said teststrips.

4. A method for obtaining full shot peening coverage of a high hardnesssteel item in a minimum amount of time comprising the steps of shotpeening a test strip having a hardness equal to that of said highhardness steel item to full coverage and then shot peening said highhardness steel item for a time equal to a predetermined multiple of thattime required 'for full coverage of the test strip.

5. A method for shot peening to full coverage in the optimum time asteel with a hardness of about RC55 or above by subjecting said steel tothe driven shot for a time expressed by the equation:

Where Ty is the shot peening time sought for the high hardness steel,

Ttsy is the shortest time at which further shot peening of a test stripof the high hardness steel results in negligible increase in arc height,

Ttsa is the shortest time at which further shot peening of a test stripunder equivalent peening conditions to those of said high hardness steeltest strip, which is dimensionally equivalent to the high hardness teststrip but which has a hardness of Rockwell C44-50, results in negligibleincrease in arc height,

Ta is the minimum shot peening full coverage time under equivalentpeening conditions to those associated with time Ty, for a steel havinga hardness'of Rockwell 044-50 and having an area equivalent to that ofthe high hardness steel-to be shot peened, and

( a) Ttsa) 6. A method for determining full shot peening coverage on ahigh hardness steel article comprising the steps of shot peening a teststrip of the high hardness steel for the shortest time required toproduce substantially the maximum arc height of said strip (Ttsy), shotpeening under equivalent conditions a dimensionally equivalent teststrip having a hardness of Rockwell C4450 for the shortest time requiredto produce the maximum arc height of said equivalent test strip (Ttsa),shot peening to full coverage to obtain a determinable time a firstarticle physically equivalent to the high hardness steel article andhaving a hardness of Rockwell 044-50 (Ta), and shot ASF=the ratio ofpeening under conditions equivalent to those of said first article thehigh hardness steel article for a time (Ty) expressed by the equation:

( y) Ttsa) 7. A method for shot peening a high hardness metal to fullcoverage in a minimum time comprising the steps of shot peening to fullcoverage a metal having a specified area and a hardness of RockwellC44-50 in the minimum time and recording the time required, shot peeningtwo dimensionally equivalent test strfps having a hardness of Rockwell044-50 and a hardness equal to that of the high hardness metal to beshot peened, respectively, to determine a greater-thanone ratio from thetime required for the two strips, and shot peening the high hardnessmetal having an area equivalent to that of said specified area, andunder equivalent peening conditions, for a time which is equal to theproduct of the time required for shot peening said metal and the ratiodetermined from the test strips.

8. A method for preventing over peening when shot peening a highhardness steel comprising the steps of shot peening the high hardnesssteel for a time expressed by the following equation:

Ty- (Ta) Ttsa) (Ta)(ASF) Ty is the shot peening time sought for the highhardness steel,

Tzsy is the time at which further shot peening of a test strip of thehigh hardness steel results in negligible increase in arc height,

Ttsa is the time at which further shot peening of a test strip, underequivalent conditions to those of said high hardness steel test stripwhich is dimensionally equivalent to the high hardness test strip butwhich has a hardness of Rockwell C44-50, results in negligible increasein arc height,

Ta is the shot peening full coverage time under equivalent peeningconditions as those associated with time Ty, for a steel having ahardness of Rockwell C44-50 and having an area equivalent to that of thehigh hardness steel to be shot peened, and

where 9. A method of shot peening a high hardness steel to full coveragein the minimum time comprising the steps of determining the respectiveminimum times required to shot peen test strips of a specific hz'ghhardness steel and of a steel having a specific hardness betweenRockwell C44-50 to substantially the maximum arc height of each, andthen shot peening the high hardness steel for a time which ismathematically derived from the minimum times required for the teststrips.

10. A method for determining the optimum shot peening time required toobtain maximum fatigue life for a high hardness steel, comprising thesteps of experimentally determining the peening time (Ta) necessary forfull coverage of steel (Sa) having a hardness in the range of RockwellC44-50, and then shot peening under equivalent conditions a highhardness steel dimensionally equivalent to Sa for a time which is amultiple of the full coverage time for Se and is equal to:

(Ttsy) Ttsa) Where Ttsy=the minimum shot peening time required to obtainsubstantially the maximum arc height for a high hardness steel teststrip, and Ttsa=the minimum shot peening time required to obtainsubstantially the maximum arc height for a test strip having a hardnessequivalent to Sa when Ttsa and Ttsy are shot peened under like peeningconditions.

No references cited.

ASF=the ratio of

